In recent years, there has been a demand for further resource savings of an image forming apparatus, and duplex printing has been increasingly utilized to meet the demand. Therefore, in an image forming apparatus having a duplex printing function, higher duplex printing speed is demanded to enhance production efficiency.
In order to attain higher printing speed in the image forming apparatus having the duplex printing function, for example, blank paper and paper printed on one side are alternately conveyed at the same interval as in a case of one-side printing. By alternately printing the blank paper and the paper printed on one side, higher printing speed can be attained. Therefore, it is necessary to reverse and convey at least two sheets of paper printed on one side in the early stage, which are to be conveyed at the start of the duplex printing, and bring the respective sheets into a standby state.
Meanwhile, a conventional image forming apparatus having a duplex printing function has the following structure. That is, when the duplex printing is performed, two sheets at the start of printing are brought into a standby state by stopping the preceding sheet at a preceding-sheet stop position and stopping the succeeding sheet at a succeeding-sheet stop position, respectively (see PTL 1).
The preceding-sheet stop position and the succeeding-sheet stop position are preset depending on an image forming apparatus to be used, and hence cannot be changed according to the size of the sheet to be printed. Therefore, in the conventional image forming apparatus disclosed in PTL 1, even in a case of a small-size sheet, the interval (printing interval) between the preceding sheet and the succeeding sheet cannot be reduced, with the result that an unnecessary printing interval is provided. Then, such an unnecessary printing interval suppresses the increase in printing speed.
In contrast, there is disclosed an image forming apparatus in which two conveying paths that allow the preceding sheet and the succeeding sheet to stand by are provided in parallel, and the printing interval (conveyance interval) is reduced by causing the respective sheets to stand by on the conveying paths (see PTL 2). However, in the conventional image forming apparatus disclosed in PTL 2, the two conveying paths are provided, and hence it is necessary to provide a structure for alternately sorting the sheets to the respective conveying paths, and a structure for alternately reconveying the sheets to an image forming portion. As a result, the apparatus becomes complex and the manufacturing cost increases. Though the image forming apparatus has been required to be downsized in recent years, the two conveying paths provided in parallel upsize the entire image forming apparatus, and hence are regarded as an obstacle to the request for downsizing.